1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, printer, or facsimile apparatus which uses electrophotography and, more particularly, to an image forming apparatus which performs image formation using a two-component developing agent.
2. Description of the Prior Art
As a developing agent for developing an electrostatic latent image formed on an image carrier, a two-component developing agent formed from a mixture of nonmagnetic toner particles and magnetic carrier particles is suitably used.
Recently, in order to realize high image quality and high durability, toner particles have been reduced in diameter, and toner with high sphericity such as polymerized toner have been used. When such toner is used, a high-resolution, high-fidelity image can be obtained. On the other hand, toner scattering and fog tend to occur. As a measure against this problem, a technique of also reducing carrier particles in diameter has been used. However, reducing the carrier particle diameter makes it difficult to mix replenished toner with a developing agent. As a consequence, scattering of insufficiently charged toner and fog tend to occur.
In order to prevent toner scattering and fog, as disclosed in Japanese Unexamined Patent Publication No. 1-166073, it is necessary to sufficiently agitate toner and a carrier and keep the toner density of a developing agent (the mixing ratio between toner particles and carrier particles) constant. For this purpose, a toner density sensor is used to detect a toner density by detecting the permeability of a developing agent agitated in a developing device, and the detected output is compared with a predetermined threshold, thereby replenishing toner.
A convey member for conveying a developing agent while agitating it with a conveyor screw is used to sufficiently agitate toner and a carrier so as to obtain a charged state by mutual friction between the toner and the carrier.
The toner density sensor is placed to oppose the convey member, which agitates/conveys a developing agent, so as to detect a toner density. To prevent toner scattering and fog, high-precision toner density control is required. However, as the carrier particle diameter decreases, the fluidity of the developing agent decreases. This has greatly degraded the substantial toner density controllability of the toner density sensor, resulting in worsening the problems of toner scattering and fog.